In directing the course of earth boreholes so that a borehole proceeds during drilling such as to arrive at the planned depth with a preferred location downhole relative to the starting point, survey instruments are commonly used. Usually the instruments are housed in a torpedo-like package, and the package is usually dropped down the drill string at the earth surface. The package descends about one thousand feet per minute in a fluid filled pipe to be stopped by a pipe bore restriction near the drill bit. The package is usually recovered when the drill string is removed from the hole. It can be recovered by a wire line down the pipe bore.
The instruments commonly consist of means to sense and to record the relationship of the instrument to the earth's gravity vector and the earth's magnetic field. Apparatus is commonly provided to position the instrument, once downhole, with a specific relationship to the drill string centerline, and sometimes to a specific radial plane of the drill string. By this process, the relationship of the drill string to the earth gravity and magnetic field vectors can, in turn, be established. To protect the instrument from hydrostatic pressure and geothermal heat, a housing is provided that is sealed and capable of containing the instrument. The housing, with its fittings and contents, is often called a survey instrument package. The housing must be of such diameter that it will pass through all restrictions encountered in the drill string bore through which it passes in going from the earth surface to the downhole location, commonly established by a restriction near the drill bit at the lower end of the drill string that the housing will not go through. Azimuthal orientation is usually accomplished by a cam arrangement known as a muleshoe and an indexing pin that engages the muleshoe. The muleshoe can be either on the instrument housing or the drill string bore as long as the indexing pin is on the other. Such systems are in widespread use and broadly found in well drilling related literature.